The phone will likely retail for around $600, based on its predecessors, Feroli wrote.

"If the imported cost component is similar to previous leading generation phones it would imply around a $200 per phone addition to imports (which is a subtraction from GDP). The difference between these two figures, $400, would represent the trade margins, which figure into GDP," he wrote

IPhone 5 sales, according to Feroli, could boost GDP by $3.2 billion, or $12.8 billion at an annual rate, increasing fourth-quarter growth by a third of a percentage point. The lift "would limit the downside risk to our Q4 GDP growth projection, which remains 2%," Feroli wrote.

"This estimate seems fairly large, and for that reason should be treated skeptically," Feroli said. "However, we think the recent evidence is consistent with this projection."

Last month, the government said U.S. economic growth in the second quarter expanded at an annual rate of 1.7% -- better than expected but still the the slowest since last summer.